I was told one thing about the "rocker" setup and did notice it on the field; when you're pushing someone, the robot gets on the back four wheels. This provides a greater amount of 'traction' or 'torque' (maybe 'power' is the right word? not sure...) because the weight is applied where the robot needs it. With traction wheels and the center wheel lowered about 1/8" (and if you can get your CG close to the middle) you should be able to make a drive train that has lots of pushing power and can turn on a dime.
Also, I don't believe it's too difficult to fabricate a pultruded fiberglass frame for a "rocker" setup. We didn't use any 'fancy' machining on it, just a reciprocating saw and drill (and some very accurate measurements!) Here are
two examples of our drive train from this year (I don't have any build pictures uploaded... sorry!)
EDIT: that 1/8" is incredibly hard (if not impossible) to see in those pictures, but the center wheel doesn't need to be much lower for the drive train to work properly.